Bone is capable of self-healing at a fracture site by the formation of callus which is able to reunite the ends of the fractured bone. Callus formation is triggered and maintained by relative movement of the fractured bone ends and occurs during a specific and limited time period following occurrence of the fracture.
If allowed to heal completely naturally, a fractured bone would heal in a poorly aligned condition, resulting in consequential future problems. Therefore the fractured bone ends are more usually manipulated into a well-aligned condition (fracture reduction) before callus formation and the natural healing process occurs. Once reduced, the fracture needs to be supported or fixed in order to maintain the desired alignment.
Rigid fixation of the fractured bone ends means that they are kept well aligned but may lead to a reduction or prevention of the formation of callus, therefore prolonging the natural healing process.
Treatment of a bone fracture by providing external support (e.g. a plaster of Paris cast) allows relative movement of the fractured bone ends to occur, which promotes callus formation. However, such external supports may not be suitable to assist with the need to accurately align the fractured bone ends, particularly with unstable or metastable fractures.
To alleviate these problems, external bone fixators have been developed which hold the fractured bone ends together sufficiently rigidly to maintain accurate alignment and yet at the same time allow sufficient relative movement between the fractured bone ends to promote callus formation. Such external fixators are applied externally to the injured limb and are attached to the fractured bone ends by bone pins or screws which pass through the soft tissue of the limb and into the bone.
A means for securing wires to a fixator is described in U.S. Pat. No. 4,978,348 (Ilizarov) wherein wires or fixing pins are secured in wire holders which consist of a bolt with plates mounted thereon and open slots in which the wires of fixing pins locate for clamping in a criss-cross manner.
An example of a ring fixator is described in WO99/60950 (Electro-Biology, Inc). This document discloses a ring-shaped external fixator which secures one or more tension wires adapted to pass through the bone. The tension wires are secured by means of an adjustable tension wire carriage of relatively complex construction.
In some cases, the end of the tension wire needs to be secured at an angle to the plane containing the ring. In such cases, articulated (and hence relatively complex) clamping elements are required. U.S. Pat. No. 6,537,275 (Orthofix Srl) discloses a simplified clamping element for securing tension wires (“bone fragment stretching wires”) to the rings of a fixator. The clamp element includes a swivel joint to enable the desired angular position to be selected. The tension wire is threaded through a hole in the clamping element and held therein by means of a grub screw which moves perpendicular to the longitudinal axis of the wire to clamp down thereon. This grub screw clamping arrangement is a well-known means of securing tension wires, not only used in the articulated clamping element of U.S. Pat. No. 6,537,275.
Tension wires are not only used in fixators of the types described above, but also in surgical orthopaedic reduction apparatus such as that described in EP0984729 (Keele University et al). The invention described herein may also be useful in any surgical applications in which tension wires need to be secured or retained.